The present invention relates to a method for processing an image obtained by imaging a pattern to thereby measure dimensions of the pattern, and particularly to a pattern measuring method suitable for evaluating, using an electron beam image of a circuit pattern, whether a processed form of the circuit pattern formed on a wafer in a semiconductor manufacturing process is good or not.
Nowadays, measurement and management of pattern dimensions using a critical dimension scanning electron microscope (length measuring SEM) dedicated to measurement is generally being performed in a semiconductor manufacturing process. The measurement of the pattern dimensions is automated by effecting an image processing technique on an acquired length measuring SEM image. Consequentially, operator's skill becomes unnecessary and variations in measurement due to differences among individuals have also been reduced. Such pattern measurement based on image processing is mainly intended for a single-layer pattern such as a resist, an insulating film, polysilicon or the like. There was also so much to make up, as shapes, relatively simple ones such as a circular form, wiring, etc.
FIGS. 2A, 2B and 2C show an example illustrative of measuring techniques. An image signal of an SEM changes according to the shape and material of a pattern and brightly glows at each edge portion of the pattern in particular. FIGS. 2A through 2C illustrates a processed example of a signal waveform indicative of a wiring shape pattern. In the signal waveform in the figures, two peaks (2011 and 2012) large in signal amount are equivalent to edge portions of a wiring. The positions of such edge portions are determined by such techniques as shown in FIGS. 2A through 2C so that the dimensions of the target pattern may be measured. The technique shown in FIG. 2A is a method (maximum gradient method) for detecting a maximum tilt position of each peak, the technique shown in FIG. 2B is a threshold method for detecting an edge position at a predetermined threshold value, and the technique shown in FIG. 2C is a linear approximate method for applying straight lines to each edge portion and a base material portion and detecting points of intersection of these.
In the pattern shown in FIGS. 2A through 2C, the peaks of the signal waveform are formed such that only one peak exists with respect to one edge. Therefore, the pattern dimensions could be measured with relative ease by such a method. With a change in recent semiconductor manufacturing techniques, however, the structure of a gate electrode is in the process of changing from such a conventional single-layer structure as shown in FIG. 3A to a multilayer film structure as in the case of a polysilicide gate or polymetal gate shown in FIG. 3B. Also the structure of device isolation changes from such a relatively simple LOCOS (Local Oxidization of Si) structure as shown in FIG. 3E to such Shallow Trench Isolation (hereinafter called “STI”: device isolation) as shown in FIG. 3F. Further, for example, such a dual damascene structure as shown in FIG. 3D in which connecting viaholes shown in FIG. 3C are simultaneously formed, also appear in a wiring process. Ones complex in shape and material is on the increase. Therefore, samples each hard to cope with such a waveform that the signal waveform corresponding to each side face of the pattern has only the single peak such as shown in FIG. 2 is on the increase.
FIGS. 4A, 4B and 4C typically shows an SEM image of an STI pattern similar to FIG. 3E. FIG. 4A is a sectional typical view, FIG. 4B is an example of an image, and FIG. 4C is a waveform of FIG. 4B. As shown in FIGS. 4A through 4C, the waveform of a sample having such complex structure and shape has a plurality of peaks adjacent to one another. Such a measuring method as shown in FIG. 2 does not often obtain a proper result. A method for designating peaks to edges having such plural peaks respectively to perform edge detection, etc. have been disclosed in, for example, Japanese Patent Laid-open Nos. 2003-173948 and 2003-243291.
As shown in FIGS. 4A through 4C, the SEM signal waveform becomes complex in the pattern complex in shape. It is difficult to stably measure a desired edge. In particular, the setting of an image processing parameter for edge detection is complex. It is difficult to set a proper parameter without knowledge about image processing. Further, a problem arises in that since the optimum value is determined by repetition of setting and measurement, time is required.
It is also unclear to which portion (such as the width of a top portion or the bottom, or the width at other position) of an actual three-dimensional form pattern the pattern edge position decided on the SEM image for measurement corresponds. A problem arises in that it is difficult to accurately evaluate a three-dimensional form of an actually formed pattern.